Heavy trucks, trailers and other commercial vehicles typically use an air brake system to provide the braking forces necessary to stop the vehicle. Such a system typically includes a brake pedal positioned on the floor of the driver's cab or compartment of the vehicle that, upon actuation, provides air from an air reservoir to an air chamber. The air chamber acts as a pneumatic actuator in that it features an actuator rod that either extends out of or retracts into the air chamber so as to activate the mechanism that pushes the brake lining material of the brake shoes against the vehicle brake drum at each vehicle wheel-end. The mechanism typically includes a slack adjustor which turns a cam roller via a camshaft so as to force the brake shoes to engage the brake drum so as to stop the vehicle.
An example of a prior art pneumatic or air chamber of such an air brake system is described in U.S. Pat. No. 5,829,339 to Smith, the contents of which are hereby incorporated by reference.
Cross-sectional views of a prior art air chamber are also provided in FIGS. 1A-1D. As explained in greater detail below, with reference to FIGS. 1A-1D, a large main compression spring 10 (also known as a parking spring or a power spring) serves as a mechanical means to prevent the vehicle from rolling when there is no air in the brake system and when the vehicle is stationary or parked. This spring supplies the parking force needed to hold the vehicle stationary. A larger or stronger spring typically means that a larger parking force can be achieved.
One problem with such a design is that a great deal of air pressure is needed to keep the main spring from applying the brake and thereby maintain the spring in a compressed state (illustrated in FIG. 1A). Also as the brake applies and the main spring 10 extends (as illustrated in FIG. 1C), one is not able to capitalize on the high amount of force that the spring exhibits in the compressed state (due to the equation Spring Force=K×X, where K is the spring constant and X is the compression distance of the spring). To this end, the main spring is subjected to very high compressive forces when in the condition of FIGS. 1A, 1B and 1D that are never translated to the parking brake force for the vehicle.
In addition, while in the compressed state (FIG. 1A) the main spring coils are close together and could be touching. This contact, combined with the vibrations experienced by the axle and vehicle as it drives, could cause an increase in wear in the spring coils. This wear could possibly break through the spring plating and damage the spring surface creating high stress areas and, without a protective coating, the spring would be subject to corrosion. The resulting rust pits become stress risers that will shorten the life of the spring.
A coil spring failure can result in a punctured diaphragm or a reduction in stroke, parking force, or the inability to completely release a brake for a given wheel-end of the vehicle. As such, much work must be put into protecting the spring from corrosion and also from individual coil contact.